1. Field of the Invention
The present invention relates to an apparatus adapted to control an operating speed of an electrically controlled device including an electrically operated member such as an electric motor, by controlling the duty cycle of an electric current to be applied to the electrically operated member.
2. Discussion of the Prior Art
In various fields of art using an electrically controlled device which includes an electrically operated member, it is usually required to control the electrically operated member so as to maintain the operating speed of the electrically controlled device exactly at a desired or target value, irrespective of a variation in the operating conditions such as a load applied to the electrically controlled device, a line voltage used, and the ambient temperature. To this end, there is widely used a speed control apparatus for controlling the operating speed of the electrically controlled device, by regulating the duty cycle of an electric current to be applied to the electrically operated member. Generally, such a speed control apparatus includes (a) an error signal generator for generating an error signal indicative of an error between an actual and a target value of the operating speed of an electrically controlled device which includes an electrically operated member, (b) a reference signal generator for generating reference signal, (c) a comparator connected to the error and reference signal generators, for comparing the received error and reference signals with each other and thereby generating an output signal according to a result of the comparison, and (d) an energy control device connected between the comparator and the electrically operated member, for controlling an energy to be supplied to the electrically controlled device, by applying an electric current to the electrically operated member such that the duty cycle of the current is changed according to the output signal of the comparator.
An example of such a speed control device is a pulse-width modulation (PWM) device or pulse-duration modulation (PDM) device used for controlling a DC motor as an electrically operated member used in a certain electrically controlled device. This pulse-width or pulse-duration modulation device generally uses a triangle generator as the reference signal generator, and uses a phase-locked loop (PLL) circuit as part of the error signal generator. The triangle generator generates a reference signal having a triangular waveform. The PLL circuit receives an output signal of a rotary encoder disposed to detect the operating speed of the DC motor, and an output signal of an oscillator, and generates an output signal indicative of a difference between the frequencies of the output signals of the rotary encoder and oscillator. In this arrangement, the operating speed of the DC motor may be maintained at a desired level if the frequency of the oscillator is held constant, or controlled to a desired level if the frequency of the oscillator can be changed as desired.
The electrically controlled device whose speed is controlled by the speed control apparatus in question is not limited to an electrically operated device which includes an electrically operated member such as an electric motor. The speed control apparatus is equally applicable to any electrically controlled device which is driven by an energy other than an electric energy. For example, the apparatus is applicable to a hydraulically operated device such as a hydraulic cylinder, which is electrically controlled by a solenoid-operated valve or valves for regulating a hydraulic pressure or a rate of flow of a working fluid in the hydraulic system. In this case, the energy supplied to the hydraulic cylinder is controlled by regulating the hydraulic pressure or fluid flow rate, and the operating speed of the hydraulic cylinder can be controlled by regulating the duty cycle of an electric current to be applied to the solenoid-operated valve or valves, i.e., the electrically operated member.
The known duty cycle control apparatus as described above is not capable of sufficiently accurately controlling the operating speed of an electrically controlled device, where the load to be applied to the device is changed over a wide range. To assure exact regulation of the operating speed to a desired level, the duty cycle of an electric current to be applied to an electrically operated member of the device should be controlled over a range which corresponds to a possible range of variation in the load to be applied to the device. Namely, the range of regulation of the duty cycle should be commensurate with the range of variation of the load. However, it is difficult to maintain the optimum operating characteristics of the components of the speed control apparatus over the entire range of variation of the duty cycle if this range is considerably wide. Accordingly, where the load varies over a relatively wide range, the known speed control apparatus is not sufficiently reliable to assure accurate regulation of the operating speed of the electrically controlled device by regulating the duty cycle of an electric current to be applied to the electrically operated member.
In view of the above drawback of the known speed control apparatus, the mass or moment of inertia of the electrically controlled device is increased to reduce the range of variation of the load to be applied to the device and thereby improve the speed control accuracy by regulation of the duty cycle of the electrically operated member. However, this solution gives rise to an increase in the cost of manufacture of the electrically controlled device.